The invention relates to an apparatus for large-area, uniform ionic etching by means of a vacuum apparatus with a tank having gas inlet and gas outlet openings, and having at least one large-area anode at ground potential and a cathode disposed substantially parallel thereto, which is provided as a substrate holder and is connected with a high-frequency voltage source.
Apparatus which are suited for ionic etching and are operated within vacuum apparatus and in which sputtering processes can also be performed, have long been known. At this point only two printed disclosures DE-22 41 229 C2 and DE-21 49 606 will be referred to, in which apparatus are described which permit substrate coating by cathode sputtering and, by a mere reversing of the polarity of the electrodes, also permit ionic etching.
In DE-22 41 229 C2, a so-called closed system is described for the ionic etching of substrates, within which a plasma forms which is necessary for ablation of material on the substrate and is largely surrounded by the electrode surfaces. No constant gas exchange takes place within the plasma volume. The maintenance of a stable plasma within the closed etching chamber, however, requires a highly precise mechanical enclosure, i.e., a highly accurate positioning of the electrode surfaces while allowing for precisely maintained spacing requirements, for one thing, and for another thing a precise establishment of the potential conditions between the grounded cathode on which the substrate to be etched is mounted, and the anode above it.
Also, for the uniform ablation of material from the substrate surface a very uniform distribution of the plasma density over the substrate is necessary. In connection with the prevailing potential gradient,, however, very different etching rates can establish themselves on the substrate surface on the basis of local differences in the parameters determining the etching process (e.g., plasma density, electrode spacing and voltage ratio).
With the known etching apparatus, as described in the above disclosures, for example, substantially uniform etching rates can be achieved on disk-shaped substrate bodies with a diameter of up to 150 mm. Larger substrate surfaces, however, cannot be treated by etching technology with the uniformity described, because a lower rate of ablation occurs, especially at the marginal areas of the substrate, than in the middle area of the substrate. This is mainly due to the fact that the plasma thins out considerably at the marginal areas of the substrate due to plasma efflux.